BIP-375: skip ineligible inputs when combining ecdh shares

add fake ecdh share and dleq proof to P2SH input for valid test: two inputs using per-input ECDH shares - only eligible inputs contribute shares (P2SH excluded)

remove unused return string from is_input_eligible

Author: macgyver13

bip-0375/bip375_test_vectors.json

@@ -1011,7 +1011,7 @@
     },
     {
       "description": "can finalize: two inputs using per-input ECDH shares - only eligible inputs contribute shares (P2SH excluded)",
-      "psbt": "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",
+      "psbt": "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",
       "supplementary": {
         "inputs": [
           {

bip-0375/validator/inputs.py

@@ -50,6 +50,8 @@ def collect_input_ecdh_and_pubkey(
     for input_map in psbt.i:
         input_ecdh = input_map.get_by_key(PSBT_IN_SP_ECDH_SHARE, scan_key)
         if input_ecdh:
+            if not is_input_eligible(input_map):
+                continue # skip ineligible inputs
             ecdh_point = GE.from_bytes(input_ecdh)
             combined_ecdh = (
                 ecdh_point if combined_ecdh is None else combined_ecdh + ecdh_point
@@ -71,7 +73,7 @@ def pubkey_from_eligible_input(input_map: BIP375PSBTMap) -> Optional[GE]:
 
     Returns a GE point (public key), or None if not found
     """
-    if not is_input_eligible(input_map)[0]:
+    if not is_input_eligible(input_map):
         return None
 
     # Try BIP32 derivation first (key_data is the pubkey)
@@ -131,32 +133,32 @@ def _parse_non_witness_utxo(non_witness_utxo: bytes, output_index: int) -> bytes
 # ============================================================================
 
 
-def is_input_eligible(input_map: BIP375PSBTMap) -> Tuple[bool, str]:
+def is_input_eligible(input_map: BIP375PSBTMap) -> bool:
     """Check if input is eligible for silent payments"""
     script_pubkey = _script_pubkey_from_psbt_input(input_map)
     assert script_pubkey is not None, (
         "scriptPubKey could not be extracted from PSBT input"
     )
 
     if not _has_eligible_script_type(script_pubkey):
-        return False, "ineligible input type"
+        return False
 
     NUMS_H = bytes.fromhex(
         "50929b74c1a04954b78b4b6035e97a5e078a5a0f28ec96d547bfee9ace803ac0"
     )
     if _is_p2tr(script_pubkey):
         tap_internal_key = input_map.get(PSBT_IN_TAP_INTERNAL_KEY)
         if tap_internal_key == NUMS_H:
-            return False, "P2TR uses NUMS point H as internal key"
+            return False
 
     if _is_p2sh(script_pubkey):
         if PSBT_IN_REDEEM_SCRIPT in input_map:
             redeem_script = input_map[PSBT_IN_REDEEM_SCRIPT]
             if not _is_p2wpkh(redeem_script):
-                return False, "P2SH is not P2SH-P2WPKH"
+                return False
         else:
-            assert False, "P2SH input missing PSBT_IN_REDEEM_SCRIPT"
-    return True, None
+            assert False
+    return True
 
 
 def _has_eligible_script_type(script_pubkey: bytes) -> bool:

bip-0375/validator/validate_psbt.py

@@ -185,22 +185,15 @@ def validate_ecdh_coverage(psbt: PSBT) -> Tuple[bool, str]:
         # Verify per-input coverage for eligible inputs
         if scan_key_has_computed_output and not has_global_ecdh:
             for i, input_map in enumerate(psbt.i):
-                is_eligible, _ = is_input_eligible(input_map)
-                ecdh_share = input_map.get_by_key(PSBT_IN_SP_ECDH_SHARE, scan_key)
-                # Disabled this check for now since it is not strictly forbidden by BIP-375
-                if not is_eligible:
+                if not is_input_eligible(input_map):
                     continue
-                # if not is_eligible and ecdh_share:
-                #     return (
-                #         False,
-                #         f"Input {i} has ECDH share but is ineligible for silent payments",
-                #     )
-                if is_eligible and not ecdh_share:
+                ecdh_share = input_map.get_by_key(PSBT_IN_SP_ECDH_SHARE, scan_key)
+                if not ecdh_share:
                     return (
                         False,
                         f"Output script set but eligible input {i} missing ECDH share",
                     )
-                if ecdh_share:
+                else:
                     # Verify per-input DLEQ proofs
                     dleq_proof = input_map.get_by_key(PSBT_IN_SP_DLEQ, scan_key)
                     if not dleq_proof: